A photoelectric sensor is a device or devices used to detect the presence of an object by using a light transmitter and a photoelectric receiver. Photoelectric sensing uses a beam of light transmitted from the light transmitter, and the presence or absence of the transmitted light arriving at the photoelectric receiver, to detect the presence or absence of an object. This technology is an alternative to other types of sensing, such as limit switches and inductive proximity sensors. Photoelectric sensing may be employed when the required sensing distances are longer, or when the item to be sensed is non-metal.
Generally, photoelectric sensors include one of three functional types. An opposed arrangement consists of a receiver located within the line-of-sight of the transmitter. In this type, light is transmitted by the transmitter directed toward the receiver, and an object is detected when the light is blocked from getting to the receiver. A retro-reflective arrangement places the transmitter and receiver at the same approximate location, and uses a reflector to bounce the light beam back from the transmitter to the receiver. An object is sensed when the light beam is interrupted and fails to reach the receiver. A proximity-sensing arrangement is one in which the transmitted radiation must reflect off of the object in order to reach the receiver. In this type, an object is detected when the receiver sees the light from the transmitter rather than when the receiver fails to see the light from the transmitter.